Piezoelectric pumps including pump bodies with pump chambers and piezoelectric elements fixed to the pump bodies so as to close openings of the pump chambers and bent by voltage application so as to change the volumes of the pump chambers are well known. Examples of such piezoelectric elements include unimorph cells and bimorph cells, and both types of cells have a disadvantage that they cannot achieve a sufficient discharge flow rate since the peripheral portions of the piezoelectric elements are fixed to pump bodies and the positions of the central portions of the piezoelectric elements cannot be significantly changed.
To solve this problem, Patent Document 1 describes a piezoelectric pump including a controllable unimorph film formed of a first layer that can be driven by a piezoelectric effect and a supporting layer joined to the first layer. The film has a peripheral area and a central area, both of them being driven by a piezoelectric effect, and is controlled such that the central area is expanded when the peripheral area is contracted in lateral directions. In this case, a large displacement can be achieved at the central portion of the film even when the film is firmly supported by a pump body at the peripheral portion thereof since directions of displacement are opposite to each other in the central portion and the peripheral portion. As a result, a high discharge flow rate can be achieved.
Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 1-500892
To solve the above-described problem, Patent Document 2 describes a piezoelectric pump including a bimorph cell serving as a piezoelectric element. The piezoelectric element has central electrodes and peripheral electrodes supported by a pump body, the electrodes being separated from each other, and AC voltages having polarities opposite to each other are applied to the central electrodes and the peripheral electrodes. In this case, a displacement larger than that achieved by the piezoelectric pump described in Patent Document 1 can be achieved since the piezoelectric element has a bimorph structure including two piezoelectric bodies bonded to each other.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 3-54383
FIG. 27 illustrates the piezoelectric element and a driving circuit for the piezoelectric element described in Patent Document 2. In FIG. 27, a piezoelectric element 100 includes two piezoelectric bodies 101 and 102 bonded to each other with a metal plate 103 interposed therebetween and electrodes formed on the top and bottom surfaces of the laminate. The electrode on the top surface includes a peripheral electrode portion 104 and a central electrode portion 105, and the electrode on the bottom surface includes a peripheral electrode portion 106 and a central electrode portion 107. One end of an AC power source 108 is connected to the metal plate (common electrode) 103. The other end of the AC power source 108 is connected to the peripheral electrode portions 104 and 106 via a controller 109, and further connected to the central electrode portions 105 and 107 via an inverter 110. The entire piezoelectric bodies 101 and 102 are polarized in the same direction as indicated by arrows P.
As is clear from FIG. 27, the metal plate 103 is at a ground potential, and the phase of a voltage applied to the peripheral electrode portions 104 and 106 is shifted from that of a voltage applied to the central electrode portions 105 and 107 by 180°. The direction of an electric field E in the central portion of each piezoelectric body is opposite to that of an electric field E in the peripheral portion of the corresponding piezoelectric body. The electric fields E acting between the metal plate 103 and the electrodes on the top and bottom surfaces can expand or contract the central and peripheral portions of the piezoelectric bodies 101 and 102. The piezoelectric bodies are contracted when the directions of the electric fields and those of polarization are the same, and are expanded when the directions of the electric fields and those of polarization are opposite to each other. As a result, the directions of the displacement of the piezoelectric element 100 become opposite to each other in the central portion and the peripheral portion as described above, and a large displacement can be achieved at the central portion of the piezoelectric element 100 even when the peripheral portion of the piezoelectric element 100 is fixed to a pump body.
Since the above-described piezoelectric element 100 includes the two fired and polarized piezoelectric bodies 101 and 102 bonded to each other with the metal plate 103 interposed therebetween, the thickness of each piezoelectric body is large, and a high driving voltage is required for a desired displacement. The high driving voltage requires a large driving circuit, which is not preferable with consideration of installation of the piezoelectric pump in, in particular, portable devices. Moreover, short-circuits may occur by migration since the potentials of the peripheral electrode portions and the central electrode portions that are adjacent to each other in the same planes differ from each other. When the size of the piezoelectric element is reduced so as to correspond to a smaller piezoelectric pump, gaps for electrically separating the peripheral electrode portions and the central electrode portions are correspondingly reduced, resulting in an increase in the risk of short-circuits. Furthermore, since the potentials of the central portion and the peripheral portion are inverted during driving, it is necessary to apply three different voltages to the intervening metal plate 103, the peripheral electrode portions 104 and 106, and the central electrode portions 105 and 107. Therefore, it is necessary to extend a plurality of wiring lines from each layer, resulting in complication of wiring and complication of the driving circuit, for example, installation of the inverter 110.